Alnus, commonly known as alder, is a genus of flowering plants in the family Betulaceae. These trees and shrubs are typically found in temperate and boreal regions, often thriving in wet or poorly drained soils. Alders are known for their distinctive catkins and can grow up to 30 meters tall, depending on the species. Alders play a crucial role in their ecosystems by fixing nitrogen in the soil, which enhances soil fertility. They are also important for wildlife, providing habitat and food for various species, including birds and insects. Some common species include Alnus glutinosa and Alnus rubra.